It is often desired in high speed and wide band magnetic tape systems to drive a tape bidirectionally while minimizing the instantaneous speed variation and providing a constant tape tension across the magnetic head. If this can be done, analog (including video and instrumentation) data and digital data can be recorded with high density and high data transfer rates. A particularly significant need for these features is encountered in some data processing applications, such as where a magnetic tape store is used as backup for a magnetic disk file. Since the introduction of so-called "Winchester" technology, workers in the art have devised various magnetic tape, floppy disk and rigid disk systems to preserve data with reference to some fixed point in time until a later backup file can be generated. This need arises because in Winchester disk systems the operating mechanism is assembled under clean room conditions in a closed structure, which is not accessible in the field for maintenance purposes. Thus in the event of head crash or other defect the disk file is immobilized, and the owner or operator of the data processing system must reconstruct the data in process by going to the backup storage and repeating all intermediate inputs and manipulations during the interim. Even though improvements in Winchester-type systems have increased the mean time before failure, backup is still needed because without it the effects on a typical system, such as a large data base system, would be virtually catastrophic.
Among the techniques for backing up a Winchester disk file are the use of an additional duplicate file, which is inordinately expensive. Video tape recorders which have adequate bandwidth have been tried. However, such records are not designed to the standards of signal reliability required for digital data systems. Floppy disks which have relatively limited storage capacity, and 1/2" or 1/4" tape cassettes or cartridges, are the most commonly used backup systems because they may be readily accumulated, stored and reused. However, the data transfer rate with most cassettes and cartridges is substantially slower than the data transfer rate from a Winchester disk so that quite complex controller mechanisms must be utilized. Furthermore, the amount of time required for complete transfer of the contents of a disk file can be excessively long, typically ranging from 5 to 7 minutes at a minimum to an order of magnitude longer at the other extreme. Some systems, known as data streamers, attempt to run a tape transport sufficiently fast and record at sufficiently high densities to accept a stream of data off the disk file virtually without interruption. Heretofore, however, these systems have been high cost units requiring substantial maintenance supervision, and they have seldom been economically justified for the backup application.
A review of magnetic tape drive technology leads only to confusion as to the type of mechanism that would best provide a high speed bidirectional system having constant tension, which enables wow, skew and flutter to be minimized while also greatly reducing the propagation of shocks along the tape and the danger of tape breakage. High speed tape systems utilizing vacuum chambers and other sophisticated devices for controlling tape tension are impractically complex and expensive for backup system applications. Peripheral reel drive systems are known, but the practical application of these has largely been limited to the instrumentation field. Some attempts have been made to simplify bidirectional drives by using one way clutches in the drive trains to tape reels, sometimes in conjunction with pulleys for providing drag on the trailing side of the tape. For the most part, however, prior art systems of this type which attempt to control tape tension through various speed differential and drag arrangements are incapable of the high performance requirements imposed on disk backup systems, usually because the tensioning arrangement is not sufficiently precise and is not sufficiently closely related to the tape drive. The lack of precision in the tensioning arrangement is principally due to changing reel diameters and short term friction stability. An example of this type of system is provided by U. S. Pat. No. 3,465,357 of Anderson in which pairs of pulleys of different diameter are commonly coupled to provide tape tension via unidirectional clutches coupling the pulleys to the tape reels. The resulting arrangement, though relatively simple and inexpensive, is not capable of meeting the high performance requirements of such applications as a disk backup system, largely due to the remote and imprecise effects of differential reel drag when related to a single capstan drive.
Examples of other interesting but ineffective tape systems for disk backup purposes are provided by U.S. Pat. Nos. 3,090,573 of Matovich, Jr., 3,806,061 of Kollar et al, 3,869,100 of Flippen, Jr., 3,809,336 of Kollar et al, 3,779,485 of Wolf et al, 3,948,464 of Hata, 3,235,194 of Willis, 4,093,149 of Shroff et al, 4,094,478 of Shroff et al, 4,095,758 of Shroff and 3,921,933 of Rotter et al. Matovich utilizes a pair of one-way clutches to drive and alternately bias a capstan in a forward and reverse drive system. Kollar et al U.S. Pat. No. 3,806,061 utilize one-way magnetic clutches to create a drag on the supply spool. Flippen employs one-way clutches and drag clutches at the reel shafts to provide tape tension, as does Kollar et al U.S. Pat. No. 3,809,336. In Wolf et al magnetic clutches are used to drive the tape reels. Hata take a different and somewhat more effective approach of driving the reels through contacting capstans, but at the expense of a complicated and cumbersome reel mounting and capstan driving arrangement. Other examples of reel driving capstans are provided by Willis and by the various Shroff patents where two servo controlled motors are used. Rotter et al use unidirectional clutches to provide a braking torque.
The art references above pertains to a wide variety of digital, audio and video applications. Although high speed bidirectional operation and constant tension are the subject of many patents, none are known which provide a satisfactory resolution of the problem for many applications. Conventional transports using hub mounted reels are designed for high acceleration and deceleration rates using compliance arms or vacuum systems, and in addition to being expensive do not provide precise speed or tension control, as well as requiring the inconvenience of tape reels.